The present invention relates to an image-forming apparatus having a control means, wherein there is a function that makes a proper image, by forming image pattern xcex1 for the detection of an image density in a non-image area, and by correcting image density and gradation of an image area, by performing feed-back of its detected density, and image-forming is made to be efficient. Incidentally, the image pattern xcex1 means a flat density pattern that is formed in a non-image area located between the regular image area and the next following regular image area, and is used for detection of image density which serves to make the quality of the regular image to be appropriate.
There is an image-forming apparatus wherein a toner image is formed on an image carrier having a charging means, an exposure means and a developing means around the image carrier, then, the toner image is transferred on an intermediate transferring member (primary transfer), and the toner image is transferred electrostatically from the intermediate transferring member to a recording medium such as a transfer sheet by a transfer roller (secondary transfer). Moreover, there is known one wherein, for correcting image density and gradation, image-related parameter is corrected and an image is made to be appropriate by making image pattern xcex1 on a non-image area on the image carrier, and by providing a sensor for detecting the image pattern xcex1 on the intermediate transferring member.
In the past, as shown in TOKUKAIHEI No. 9-204108, the above-mentioned sensor is provided at a position that is between a downstream side of a primary transfer position and an upstream side of a secondary transfer position, on the intermediate transfer member, however, as it is necessary to secure the distance between the first transfer and the secondary transfer to a certain extent or more, it disturbs the miniaturization of the apparatus and the reduction of first-print-out time.
On the other hand, when the above-mentioned sensor is provided on the downstream side of the secondary transfer, it is necessary to apply bias voltage of the same polarity with that of toner on the secondary transfer roller, when the image pattern xcex1 passes through the position of the secondary transfer roller so that the above-mentioned image pattern xcex1 may not transfer to the secondary transfer roller as mentioned in TOKUKAIHEI No. 7-253729. In this case, it is thought that the back side of the transfer sheet is soiled by a soiled surface of the secondary transfer roller, and it is also thought that the density cannot be detected correctly because of disturbance of the image pattern xcex1, since an amount of toner moving to the secondary transfer roller is not zero, and the transferring amount varies depending upon environment.
To solve these problems, it is thought to let the secondary transfer roller be away from the intermediate transfer member, when the image pattern passes through the secondary transfer position, however, if it is intended to make the above-mentioned pattern xcex1 between images in the course of the continuous printing, the image is influenced by the uneven rotation of the intermediate transfer member, when the secondary transfer roller is pressed or released. As a means to prevent disturbance of the image pattern xcex1, it is thought to constitute the secondary transfer section with the non-contact transfer means such as corotron, which, however, has demerits that an ozone generating amount is increased and sheet transfer characteristics is worsened.
An object of the present invention is to provide an image-forming apparatus wherein a means for raising the print productivity and an image correcting means for improving image quality are provided, and considering the furnishing position of the sensor, bad influence is not given to the image, when the secondary transfer roller is released from or pressed on the intermediate transfer member while the image pattern xcex1 passes through the secondary transfer position, and the position of the density detection sensor to be set is considered, and less hazardous material such as ozone is emitted.
The object stated above is attained by the technical means (1) shown below.
(1) An image-forming apparatus, in which a plurality of image carrier units each having charging, exposing and developing means around itself are provided, a toner image is formed on the above-mentioned each image carrier, each toner image is superposed to be primarily transferred onto the intermediate transfer member by the transfer roller, and then, toner images are secondarily transferred collectively from the intermediate transfer member to the transfer material by the transfer roller, while, image pattern xcex1 is formed on a non-image area on the image carrier, then, is primarily transferred onto the intermediate transfer member, and its image density is detected by the image pattern xcex1 detecting sensor provided at the downstream side of the secondary transfer position in the running direction of the intermediate transfer member to face the surface of the intermediate transfer member, and there is provided a control means that conducts process control by changing image-forming conditions with the detected value of the above-mentioned detecting sensor, wherein the control means controls so that the latent-image formation of the following print image may start, after the pressure contact of the secondary transfer roller to the intermediate transfer member is released, when image pattern xcex1 has passed through the first transfer position of its final color to the direction of the downstream side of the rotation direction of the above-mentioned intermediate transfer member, and after the secondary transfer of image which is formed before the formation of the image pattern xcex1 is completed before the arrival of the image pattern xcex1 at the secondary transfer position, and after the pressure contact of the secondary transfer roller to the intermediate transfer member is applied again, immediately after the image pattern xcex1 passes through the secondary transfer position.
That is, before the image pattern xcex1 reaches the secondary transfer position after having passed through the first transfer position, and immediately after the secondary transfer of the image formed before the formation of the image pattern xcex1 was completed, the secondary transfer roller is released from pressure contact, and immediately after the image pattern xcex1 passed through the secondary transfer position, the secondary transfer roller is subjected to pressure contact again, and after that, the next image formation is performed. Due to this, it is possible to miniaturize the apparatus and to raise the productivity of prints as far as possible, using the contact type secondary transfer method in which ozone emission amount is low, under the condition that both of an image and image pattern xcex1 in the non image area are not disturbed.